1. Field of the Invention
The present invention relates to an optical pickup compatible with a recordable compact disk (CD-R) and a digital video disk (DVD), and more particularly, to an optical pickup which can record and reproduce signals with respect to a DVD and a CD-R.
A disk, a card or a tape may be used as a recording medium for recording and reproducing information such as video, audio or data at high density. Among them, a disk-shaped recording medium is mainly used. Recently, products in the field of optical disk devices have been developed from laser disks (LDs) and compact disks (CDs) to DVDs. Such optical disks comprise a plastic or glass medium having a certain thickness for transmitting a beam of light incident from an optical pickup, and an information recording surface located on the medium and on which information is recorded.
A high-density optical disk system developed up to now enlarges a numerical aperture of an objective lens and uses a relatively short-wavelength light source of 635 nm or 650 nm, to heighten a recording density. As a result, the optical disk system can record and reproduce information with respect to a DVD and can also reproduce information recorded on a CD. The high-density optical disk system uses a light source outputting light whose wavelength is 780 nm, to compatibly use a recordable compact disk (CD-R), which is a recent type of a CD. This is due to a recording characteristic of the CD-R. It is a very essential technique for compatibility between a DVD and a CD-R to enable a single optical pickup to use both the light having a wavelength of 780 nm and the light having a wavelength of 650 nm. An existing optical pickup which is compatible with a DVD and a CD-R is described below with reference to FIG. 1.
FIG. 1 shows an optical pickup which uses a single objective lens, and two laser diodes as light sources for a DVD and a CD-R, respectively. The optical pickup shown in FIG. 1 uses laser light whose wavelength is 635 nm during reproduction with respect to a DVD, and uses laser light whose wavelength is 780 nm during recording and reproduction with respect to a CD-R. A light source 1, which is a laser diode, emits a beam of light having a 635 nm wavelength which passes through a collimating lens 2 and a polarization beam splitter 3, and then proceeds to an interference filter prism 4. A light source 11, which is a laser diode, emits a beam of light having a 780 nm wavelength which passes through a collimating lens 12 and a beam splitter 13, and then proceeds to a converging lens 14. The converging lens 14 makes the beam of light incident from the beam splitter 13 converge into the prism 4. An optical system having such a structure for converging the beam of light whose wavelength is 780 nm is called a "finite optical system." The prism 4 transmits the 635 nm wavelength light which is incident after being reflected by the polarization beam splitter 3, and reflects the light beam converged by the converging lens 14. As a result, the light emitted from the light source 1 is incident to a quarter-wave plate 5 in the form of a light beam made parallel by the collimating lens 2, and the light emitted from the light source 11 is incident to the quarter-wave plate 5 in the form of a light beam diverged by the converging lens 14 and the prism 4. The light transmitted through the quarter-wave plate 5 is incident to an objective lens 7.
The objective lens 7 which is designed to have a focus on an information recording surface of a DVD 8 having a thickness of 0.6 mm, focuses the 635 nm wavelength light beam emitted from the light source 1 on the information recording surface of the DVD 8. As a result, the light reflected from the information recording surface of the DVD 8 contains the information recorded on the information recording surface. The reflected light is transmitted through the polarization beam splitter 3 and is incident to an optical detector 10 for detecting optical information.
In the case that the above-described finite optical system is not employed, when the beam of the 780 nm wavelength light emitted from the light source 11 is focused on the information recording surface of a CD-R 9 having a thickness of 1.2 mm by using the objective lens 7, spherical aberration occurs due to a difference between the DVD 8 and the CD-R 9 in thickness. The spherical aberration is due to the fact that the information recording surface of the CD-R 9 is located at a farther place from the objective lens 7 than that of the DVD 8. To reduce the spherical aberration, the finite optical system constituted by using the converging lens 14 is required. By use of a variable aperture 6 to be described later with reference to FIG. 2, the beam of the 780 nm wavelength light is formed as an optical spot of an optimized size on the information recording surface of the CD-R 9. The beam of the 780 nm wavelength light reflected from the CD-R 9 reflects from the prism 4, transmits through the converging lens 14, and reflects from the beam splitter 13, and then, is incident to a photodetector 15. The photodetector 15 detects information from the light incident from the beam splitter 13.
The variable aperture 6 shown in FIG. 1 has a structure of a thin film which can selectively transmit the incident light, with respect to a region identical to a diameter of the objective lens 7, that is, a region having a numerical aperture (NA) not more than 0.6 shown in FIG. 2. In other words, the optical surface of the variable aperture 6 is divided into a region "1" which transmits both the 635 nm wavelength light and the 780 nm wavelength light and a "region 2" which totally transmits the 635 nm wavelength light and totally reflects the 780 nm wavelength light, on the basis of the center of the optical surface. The region 1 is an area of the numerical aperture (NA) not more than 0.45. The region 2 is an outer area of the region 1 and is made by coating a dielectric thin film thereon. The region 1 comprises a quartz (SiO.sub.2) thin film to remove optical aberration generated by the dielectric thin film coated region 2. Using the variable aperture 6, the 780 nm wavelength light which is transmitted through the region 1 of the numerical aperture (NA) not more than 0.45 is formed as an optical spot optimized to the information recording surface of the CD-R 9. Thus, the optical pickup of FIG. 1 can record and pick up information with respect to a CD-R 9, even when a loaded disk is changed from the DVD 8 to the CD-R 9.
However, the above-described optical pickup of FIG. 1 should achieve a finite optical system with respect to the 780 nm wavelength light, to remove the spherical aberration occurring due to the compatibility between a DVD and a CD-R. Also, due to the dielectric thin film which is formed in the region 2 having the numerical aperture (NA) not less than 0.45 of the variable aperture 6, an optical path difference occurs between the light passed through the region 1 and the light passed through the region 2. To remove this optical path difference, the variable aperture 6 requires a particular optical thin film such as a quartz thin film formed on the region 1. For this reason, a quartz coating is formed on the region 1 and a multi-layered thin film is formed on the region 2, which causes a complicated manufacturing process. Also, the thickness of the thin film is adjusted on a .mu.m scale, which does not fit mass-production.